1. Field of the Invention
This invention relates to an improved dry physical development photothermographic element for a dry thermal silver-dye bleach process. In one of its aspects it relates to such a photothermographic element containing a silver halide complexing concentration of a certain silver halide complexing agent. In another of its aspects it relates to a dry activator element for producing a dye image by means of a dry physical development thermal silver-dye bleach process. A further aspect of the invention relates to a process of thermally dye bleaching an image in a dry physical development photothermographic element with a dry activator element.
2. Description of the State of the Art
It is well known to produce a silver image in a heat developable photographic material. Heat developable photographic materials are also known as photothermograhic materials. Heat developable photographic materials after imagewise exposure are heated to moderately elevated temperatures to provide a developed image without the need for separate processing solutions or baths. The heat development can provide a developed silver image in the material.
Many methods and materials for producing positive color images with photographic silver halide are also known. Methods which are known for producing color images include the silver-dye bleach process which has been described in several publications. Other processes for forming color images include reversal processes which involve the color development of photographic silver halide elements containing incorporated color-forming couplers. In each of these processes lengthy solution processing techniques have been required which rely heavily upon precision control and sophisticated processing techniques to produce color images. It has been desirable to avoid the need for processing solutions and baths in the silver dye-bleach process.
The silver-dye bleach process involves developing a silver image in an exposed photographic silver halide emulsion containing bleachable dye. After development of the silver image, bleaching of the dye is effected in those areas where the silver image has been developed. All the silver ion is removed or rendered transparent and insensitive to light by the bleach action leaving a positive dye image in the areas in which no metallic silver was present.
Thermal silver-dye bleach materials and processes have been proposed to overcome the problems encountered in solution processing involving photographic silver-dye bleach materials. Such thermal silver-dye bleach materials and processes are described in, for example, Research Disclosure, April 1976, pages 30-32, Item 14433 of Oftedahl, Mowrey and Humphlett, published by Industrial Opportunities Ltd., Homewell, Havant, Hampshire, P09 1EF, UK; and Research Disclosure, December 1976, pages 14-15, Item 15227 of Wu. One of the requirements of silver-dye bleach materials, including thermal silver-dye bleach materials, has been the need for an acid pH to produce the desired bleaching action. Typically, the pH of the materials in the past has been required to be less than 6.0 with a silver-dye bleach catalyst and in most cases less than 4.0. This low pH has caused increased decomposition of some of the components in the silver-dye bleach materials and caused reduced storage stability as well as difficulty in handling the very acidic materials.
It has also been desirable to provide a silver-dye bleach material useful for thermal processing, which avoids the need for a silver-dye bleach catalyst. A further problem which has been encountered involves the need for a wider range of effective dyes which are useful in the silver-dye bleach process, especially the thermal silver-dye bleach process. This is especially the case with dyes which are not irreversibly reduced to colorless materials. The thermal dye bleach materials in many cases provided suitable dye images; but, the materials have not provided suitable answers to these problems.
A further silver-dye bleach material has been proposed in U.S. Pat. No. 3,414,411 of Michael et al, issued Dec. 3, 1968. This material described in this patent is an "in-camera" type system in which a photographic element comprises a support having thereon a silver halide emulsion containing the salt of an acid and a developed silver image having in association therewith a bleachable dye or dye precursor. The exposed photographic emulsion is contacted with a viscous alkaline processing solution and with a web having a component which is capable of exchanging hydrogen ion with the cation of the salt of the acid present in the emulsion. The acid lowers the pH of the emulsion to a level at which imagewise bleaching of the dye can occur in areas where metallic silver and a silver complexing agent are present. The dye bleaching is conducted in the presence of a silver-dye bleach catalyst. This material encounters the problem described, such as the need for a strong acid component to produce the desired lower pH. In addition, the processing solution of Michael et al requires the use of salts which upon drying render the coating opaque due to crystallization. As a result, the web must be delaminated from the element for viewing of the image produced, according to Michael et al.
A continuing need has existed to provide a dry physical development photothermographic element for a dry thermal silver-dye bleach process and a dry activator element for producing a dye image by means of a dry physical development thermal silver-dye bleach process without the need for a thiourea silver halide complexing agent or similar silver halide complexing agent. Such elements should (1) avoid the need of providing an acid pH, such as an acid pH of 4.0 or less, and (2) avoid the need for a separate silver-dye bleach catalyst.